1. Field of the Invention
The present invention relates to a surface treated steel sheet, for fuel containers, having excellent corrosion resistance, formability and weldability and, more particularly to a surface-treated steel sheet which, owing to its excellent formability, weldability and corrosion resistance with respect to automobile fuel and external surface corrosion environments, is suitable as a material for a container for holding and retaining an automobile fuel such as gasoline, alcohol, gasoline containing alcohol, or the like, i.e., a fuel tank, and fuel tank peripheral components.
2. Description of the Related Art
Conventionally, a terne-plated steel sheet comprised of steel sheet coated with a Pb--Sn alloy containing 3-20% of Sn has been used for gasoline tanks. The terne-plated steel sheet has good corrosion resistance against gasoline fuels and also resists corrosion by the water, sulfur and other impurities unavoidably contained in such fuels. It can also withstand the severe press forming needed to form the steel sheet into the shape of the fuel tank, and is good in weldability.
However, terne-plated steel sheet does not have fully adequate corrosion resistance against extremely corrosive fuels such as alcohol fuels, blended fuels of alcohol and gasoline, and fuels containing acids arising from fuel deterioration, formic acid, acetic acid, and/or other organic acids. In recent years, moreover, tightening of environmental regulations and, in particular, restrictions on Pb dissolution from scrapped vehicles, has created a demand for Pb-free steel sheet.
In response to the need for steel sheet with excellent corrosion resistance with respect to alcohol fuels, blended fuels of alcohol and gasoline, and other fuels with extremely severe corrosivity, JP-A-(unexamined published Japanese patent application)58-45396 discloses a steel sheet whose surface is coated with a Ni alloy containing 5-50% of Zn to a thickness of 0.5-20.mu.m. JP-A-60-121295, on the other hand, discloses a steel sheet for fuel containers obtained by electroplating the surface of a sheet with a Zn--Ni alloy containing 5-30% of Ni and further forming a Sn-coating layer thereon.
However, the corrosion resistance of the coating layers obtained by these technologies is degraded by the presence of pinholes. Moreover, during press forming, pinholes and other defects that form in the coating develop into cracks that degrade the corrosion resistance of the coating layer. Total elimination of such coating defects is difficult both technically and economically.
JP-A-62-27587 discloses a steel sheet characterized in that a first coating layer composed of Zn or Zn--Ni alloy is formed on the steel sheet surface and a Ni surface layer is formed on the first coating layer. This steel sheet is excellent in corrosion resistance against alcohol fuels and blended fuels of alcohol and gasoline.
When a steel sheet coated with a low-melting point metal such as Zn is resistance-welded using a copper electrode to pass current, it may incur cracks at the welded metal surface under conditions of high welding current. This is thought to be attributable to liquid metal embrittlement caused by a low-melting point metal such as copper or zinc. Prevention of cracking is necessary in order to improve productivity by expanding the range of production conditions and also in order to enhance the reliability of the component. JP-A-62-27587 is completely silent regarding such cracking and discloses nothing regarding prevention. Moreover, it was found that adding 5% or less of Co to the surface coating metal had absolutely no effect on susceptibility to cracking.